In the background art, there is known an exhaust purification system including an SCR that selectively reduces and purifies NOx contained in an exhaust gas using ammonia (hereinafter, referred to as NH3) as a reducing agent, the NH3 being generated by hydrolysis from urea water. In such an exhaust purification system, the injection amount of urea water is feedback-controlled such that a value of NOx contained in an exhaust gas becomes a target value based on a sensor value of an NOx sensor provided at an exhaust downstream side of the SCR (for example, see Patent Literatures 1 and 2).